System and method for quantifying x-ray backscatter system performance

ABSTRACT

A system for quantifying x-ray backscatter system performance is disclosed. The system includes one or more x-ray backscatter detectors, an x-ray tube, a support, and a plurality of rods mounted on the support and arranged in groups. Each group of rods includes at least two rods having the same width. The system also includes a user interface configured to connect to the x-ray backscatter detectors to receive a backscatter signal from the x-ray backscatter detectors associated with the x-ray tube, where the user interface plots a modulation transfer function representing x-ray backscatter for each rod of the plurality of rods from x-rays transmitted by the x-ray tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/184,644 filed on Jun. 16, 2016, which is a continuation of U.S. application Ser. No. 14/269,930, filed on May 5, 2014.

STATEMENT OF GOVERNMENT RIGHTS

The disclosure was made with United States Government support under Contract No. N00019-11-G-0001 awarded by the Department of Defense. The United States Government has certain rights in this disclosure.

TECHNICAL FIELD

The disclosure relates to devices for measuring the performance of x-ray imaging systems and, more particularly, to devices and methods for quantifying the performance of x-ray backscatter imaging systems.

BACKGROUND

It is often necessary to inspect structural components of large objects, such as aircraft, maritime vessels, automobiles, and other large investment assemblies, for defects and damage. Other structural objects that may require inspection include petrochemical facilities, power generation facilities, nuclear facilities, water treatment facilities, and the like.

Inspection of such structures and facilities by partial or complete disassembly of the structures to visually inspect internal components of interest may be impracticable because it is too time-consuming and expensive. In some instances, it may not be possible to inspect the desired component without partially destroying the component of interest. Further, transportation of such structures and components of large object or facilities to an inspection location may be difficult, expensive, and in some instances impossible.

A technique for inspecting such components is the use of x-ray imaging. Inspection by x-ray imaging requires an x-ray transmitter that transmits x-rays sufficiently powerful to pass through the object of interest and its surrounding components to be detected by a film or other detection means. Such devices are large, cumbersome and relatively expensive.

X-ray backscattering systems provide an inspection process in which x-rays are reflected from the object or component of interest and recorded by a detector or detectors. X-ray backscattering systems do not need to be powerful enough to transmit x-rays entirely through the component of interest and its surrounding components. Rather, partial penetration is all that is required. X-ray backscatter inspection systems are smaller, more portable, and less expensive than traditional x-ray imaging systems.

A problem with x-ray backscattering systems is that the reference standard used for calibrating or adjusting traditional through-transmission film or digital x-rays does not work for x-ray backscatter systems. Scattered x-rays off of the standards or gauges used for traditional through-transmission x-ray inspection do not provide relevant spatial frequency information because x-rays are scattered differently (i.e., with respect to intensity and angle) from different types of materials. Accordingly, there is a need for a method and system for quantifying x-ray backscatter system performance.

Further, in order to image very small features or detect small cracks, such as stress corrosion cracks, the size of the openings in the aperture of an x-ray backscatter device must be very small, on the order of 0.25 millimeters (mm) or 0.5 mm in diameter. However, with such small apertures, the amount of x-ray flux that is emitted from the x-ray tube is very low, which requires a relatively slow x-ray tube scan speed and a relatively small scan area for each raster scan, so that many more raster scans are required for a given inspection area.

Often, x-ray tubes with relatively larger apertures may be used for general scanning because they permit higher x-ray flux, which enables a relatively larger area to be scanned for each pass, and at a faster scan rate. However, switching between large and small apertures takes time and requires multiple sets of apertures of different sizes. Another disadvantage is that smaller apertures are more difficult and expensive to fabricate than larger apertures. The dimensions of small apertures make reproducibility of apertures difficult and require software normalization to achieve consistent flux intensities from the x-ray tube.

SUMMARY

The disclosure describes a system and method for quantifying x-ray backscatter system performance that overcomes the disadvantages of prior systems for testing x-ray backscatter system performance, especially prior test systems adapted from calibrating or adjusting traditional through-transmission film or digital x-rays. The disclosed system is highly portable and therefore can be transported to the site of the apparatus under test.

In one embodiment, a system for quantifying x-ray backscatter system performance is disclosed. The system includes one or more x-ray backscatter detectors, an x-ray tube, a support, and a plurality of rods mounted on the support and arranged in groups. Each group of rods includes at least two rods having the same width. The system also includes a user interface configured to connect to the x-ray backscatter detectors to receive a backscatter signal from the x-ray backscatter detectors associated with the x-ray tube, where the user interface plots a modulation transfer function representing x-ray backscatter for each rod of the plurality of rods from x-rays transmitted by the x-ray tube.

In another embodiment, a method for quantifying x-ray backscatter system performance is disclosed. The method includes receiving, by a user interface, a backscatter signal from one or more x-ray backscatter detectors, where the backscatter signal indicates x-ray backscatter for a plurality of rods mounted on a support and arranged in groups. Each group of rods include at least two rods having the same width. The method further includes plotting, by the user interface, a modulation transfer function representing the x-ray backscatter for each rod of the plurality of rods from x-rays transmitted by an x-ray tube.

Other objects and advantages of the disclosed system and method for quantifying x-ray backscatter system performance will be apparent from the following description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of the system for quantifying x-ray backscatter system performance;

FIG. 2 is a front elevation of the modulation transfer function standard of the system of FIG. 1;

FIG. 3 is a display of a backscatter image of the modulation transfer function standard of FIG. 2;

FIG. 4 is a display of a second backscatter image of the modulation transfer function standard of FIG. 2;

FIG. 5 is a display of a backscatter image of the modulation transfer function standard from two detectors of the system of FIG. 1, utilizing a graphical user interface of the system of FIG. 1;

FIG. 6 is a display of the line profile and peak-to-peak data displayed on the graphical user interface of FIG. 5;

FIG. 7 is a display of the modulation transfer function plot of the line profile of the display of FIG. 6;

FIG. 8 is a flowchart of the method for quantifying x-ray backscatter system performance performed by the system of FIG. 1;

FIGS. 9A and 9B show side elevation in section and end elevation, respectively, of an aperture wheel having the disclosed adjustable apertures;

FIG. 10 is a side elevation in section of another embodiment of an aperture wheel having the disclosed adjustable apertures;

FIG. 11 is a detail showing an embodiment of the disclosed adjustable aperture;

FIG. 12 is a detail showing another embodiment of the disclosed adjustable aperture;

FIGS. 13A and 13B show a plan view and a side elevation, respectively, of the adjustable aperture of FIG. 12 mounted on an end of a spoke of the aperture wheel of FIGS. 9A and 9B;

FIGS. 14A and 14B show a plan view and a side elevation, respectively, of another mounting arrangement of the adjustable aperture of FIG. 12, mounted on the end of a spoke of the aperture wheel of FIGS. 9A and 9B;

FIGS. 15A, 15B, 15C, and 15D show plan views of another embodiment of the disclosed adjustable aperture mounted in an end of a square collimator;

FIGS. 16A, 16B, 16C, and 16D show plan views of another embodiment of the disclosed adjustable aperture mounted in an end of a round collimator;

FIG. 17 shows a plan view of another embodiment of the disclosed adjustable aperture, in which nano motors position two sets of shutters; and

FIGS. 18A and 18B show plan views of another embodiment of the disclosed adjustable aperture, in which piezoelectric elements position two sets of shutters.

DETAILED DESCRIPTION

The system 10 for quantifying x-ray backscatter system performance is shown in FIG. 1. The system 10 includes a modulation transfer function (“MTF”) standard 12 and a display 14 that includes a user interface, such as a graphical user interface (“GUI”), which is referred to as a user interface 16. The display 14 receives a signal from one or more x-ray backscatter detectors 18, 20, which in an embodiment are solid state detectors, associated with an x-ray tube 22, which in an embodiment is a small filament micro-focus x-ray tube. An x-ray tube 22 in the form of a small filament micro-focus x-ray tube is used some applications because a small filament micro-focus x-ray tube is lighter and smaller in size than the x-ray tube of a conventional x-ray backscatter system, possesses a relatively smaller x-ray field of view 24, and is relatively low in radiation leaking. The x-ray tube 22 is connected to a source of electrical power 26 by power cables 28. In an embodiment, power cables 28 supplies power to the x-ray backscatter detectors 18, 20 and/or to display 14. The display 14 receives signals from the x-ray backscatter detectors 18, 20 over cables 30, 32, respectively.

As shown in FIGS. 1 and 2, the Reference standard 12 includes a support 34 in the form of a frame 55, which in an embodiment is made of aluminum channel, and in a specific embodiment square aluminum channel. A plurality of rods 36 are mounted on the support 34 and arranged parallel to each other. Each individual rod 56 of the plurality of rods 36 have generally curved outer surfaces, and are arranged in groups 38, 40, 42, 44, 46, 48, 50, 52, 54 of varying widths. In an embodiment, the rods 56 are arranged in groups 38-54 in which the rods 56 have a common width. Thus, the five rods 56 comprising group 38 are of a common width, the five rods comprising group 40 has a common width that is different from the width of the rods in group 38 (in an embodiment, the width are less than the rods of group 38), the five rods comprising group 42 has a width that is different from the widths of the rods in groups 40 and 38 (in an embodiment, the width are less than the rods of group 40), the five rods comprising group 44 have a width that differs from the widths of the rods in groups 42, 40 and 38 (in an embodiment, the width are less than the width of the rods in group 42), and so on for the remaining groups 46, 48, 50, 52, 54.

In embodiments, groups 38-54 have greater or fewer rods 56 of a common diameter in each group than the five rods per group depicted in FIG. 2. In other embodiments, the numbers of rods 56 varies from group to group in the groups 38-54. In still other embodiments, the Reference standard 12 includes groups (not shown) of rods 36 in addition to the groups 38-54 that are thicker or wider than the rods of group 38, thinner or narrower than the rods of group 54, or both.

In an embodiment, the rods 36 are arranged in parallel in a common plane on the support 34. In an embodiment, the rods 36 are curved rods that are round or circular in cross-section. In an embodiment, the rods 36 are made of a polymer or other hydrocarbon material, such as nylon 12. Use of rods 36 that are made of a polymer and that are round or circular in cross-section are preferable because the composition and shape may provide maximum backscattering from the Reference standard 12. As shown in FIG. 2, in an embodiment, the rods 56 of each of the groups 38-52 of the plurality of rods 36 are spaced evenly from each other. In an embodiment, the rods 56 of each of the groups 38-54 are spaced from each other a distance equal to, or approximately equal to, the width of the rods in each of the groups.

As shown in FIG. 2, the support 34 of the Reference standard 12 includes a frame 55 having a pair of opposing side rails 57, 58 and a pair of opposing end rails 60, 62. In an embodiment, the frame 55 are generally rectangular, such that the side rails 57, 58 are generally parallel to each other, and the end rails 60, 62 are parallel to each other. The rods 56 are mounted on the frame 55 to extend between the pair of opposing side rails 57, 58. In an embodiment, the rods 56 are secured within the frame 55 by set screws 64 so that the rods are properly tensioned on the frame. In a particular embodiment, the spaces 66-68 between each end rail 60, 62, respectively of the pair of opposing end rails and a next adjacent one of the rods 70, 72, respectively, is greater than a spacing between the rods 36. This increased separation may avoid backscatter caused by the end rails 60, 62.

Although the Reference standard 12 are positioned at an angle relative to the x-ray tube, it is preferable to position the MTF standard such that the support 34 positions the plurality of rods 36 perpendicular, or substantially perpendicular, to the x-ray field of view 24 transmitted by the x-ray tube 22. This are effected by mounting the support 34 on a stand 69, which in embodiments are adjustable to raise and lower the Reference standard 12 as well as pivot the standard relative to the x-ray tube 22 and the x-ray backscatter detectors 18, 20. In an embodiment, the rods 56 on the support 34 are arranged in a sequence progressing from rods having a relatively large thickness, such as the rods of group 38, to progressively thinner rods in groupings 40-54, in which each of the next grouping of rods has a thinner diameter than the rods to the right of it (as shown in FIG. 2). Thus, the groups 38-54 are arranged on the support 34 in a sequence progressing from a relatively large thickness to a relatively small thickness.

The display 14 is configured to be connected to receive a backscatter signal from x-ray backscatter detectors 18, 20 associated with the x-ray tube 22 and generate a display on user interface 16 representing photon counts of x-ray backscatter for each rod 56 of the plurality of rods 36 from x-rays transmitted by the x-ray tube 22.

As shown in FIG. 3, the user interface 16 displays a backscatter image 74 that is received from either one of the x-ray backscatter detectors 18, 20 (FIG. 1). The x-ray backscatter images from the rods 56 of group 38 (FIG. 2) are shown as group 38A, the backscatter from the rods of group 40 are shown as grouping 40A, the rods of group 42 are shown as group 42A, and so on for backscatter images 44A, 46A, 48A, 50A, 52A and 54A, which correspond to groups 44-54, respectively. The x-ray backscatter images for the thicker rods, such as the rods of groups 38, 40, 42 appear on the backscatter image 74 relatively sharply defined as groupings 38A, 40A and 42A. In contrast, the groupings of the progressively thinner rods of 44-54 (FIG. 2) are relatively less defined, and are faint and blurry, such as for backscatter images 44A, 46A, and 48A, whereas the backscatter images 50A, 52A and 54A may appear quite faint. Thus, the Reference standard 12 provides a spectrum over which the sensitivity of the x-ray backscatter detectors 18, 20 is measured. In an embodiment, the image of FIG. 3 is stored in storage 76 connected to display 14 (FIG. 1).

The backscatter image 74 is stored in storage 76 as a reference image. As shown in FIG. 4, image 77 shows x-ray backscatter images of groups 38B, 40B, 42B, 44B, 46B, 48B, 50B, 52B, and 54B corresponding to x-ray backscatter from groups 38-54 of rods, respectively of the Reference standard 12. Images 38B-54B are fainter and less sharp than their counterpart images corresponding to groups 38A-54A of the backscatter image 74. Backscatter images for groups 48B-54B is barely visible, indicating a much lower photon count detected by x-ray backscatter detector 18 and/or 20. This indicates an out-of-adjustment or out of alignment condition for the x-ray tube 22 and the x-ray backscatter detectors 18, 20 (FIG. 1).

As shown in FIG. 5, the user interface 16 (FIG. 1) of the display 14 shows a screenshot of the x-ray backscatter image 78 of the Reference standard 12 that is comprised of scans from both x-ray backscatter detectors 18, 20. The image shows the combined backscatter images of the group 38 of rods 36 (FIG. 2), shown as group 38C, the combined images of group 40 shown as images 40C, and so on for groups 42-56, shown as images 42C, 44C, 46C, 48C, 50C, 52C, and 54C. Again, the images corresponding to groups 38C-54C may be fainter and less sharp than the groups 38A-54A of backscatter image 74 of FIG. 3.

As shown in FIG. 6, the user interface 16 is configured to show a screenshot that includes an image 80 of a line profile 82 representing photon counts of the backscatter images of, for example, the backscatter image 74 in FIG. 3. The line profile 82 thus shows the photon counts for the x-ray backscatter from rods 36 of group 38 (FIG. 2) as the peaks 38D, the photon counts for group 40 as peaks 40D, and so on for groups 42-54 as peaks 42D, 44D, 46D, 48D, 50D, 52D, and 54D, respectively. The line profile 82 thus shows the ability of the x-ray backscatter detectors 18, 20 (either singly or in combination) to distinguish the individual rods for the groupings of rods of progressively decreasing thickness. From the line profile 82, the photon counts progressively decrease from the thicker groupings 38, 40, 42, 44 (FIG. 2) and become less distinct in the groupings 46, 48, 50, 52, 54, as evidenced by their corresponding line peaks 38D-54D. Groups represented by curves 48D-54D are considered indistinguishable.

Also as shown in FIG. 6, the user interface 16 includes peak-to-peak text data in display 84, listing frequency values 86, MTF frequencies 88, MTF standard deviations 90, and optionally, provide a reference number in column 92 for each value. The user interface 16 also includes virtual buttons that are actuated by a mouse for switching from one image to another, such as button 94 for reverting the user interface, peaks button 96 for finding the peaks on the line profile 82, button 98 for selecting a region of interest (“ROI”) of the curve of the entire x-ray backscatter image of FIGS. 3-5, button 100 for saving the project in storage 76 (FIG. 1), button 102 for loading a reference image (such as the image of FIG. 3), and button 104 for saving the reference image.

The user interface 16 also includes a virtual button for adding an MTF 106, removing the MTF with button 108, marking the reference with button 110, unmarking the reference with button 112, adding an MTF point with button 114 and exporting the data with button 116. Further, the image shown in FIG. 6 are selected by selecting the virtual tab 118 marked “Profile” at the top of the page. In contrast, the tab marked “Raw Data” 120 are used to call up the images of FIGS. 3, 4 and 5.

As shown in FIG. 7, the user interface 16 includes the display 14 that includes a plot 122 of the current MTF displayed on the user interface of image 80 of FIG. 6. The plot 122 is used to record, monitor and compare x-ray backscatter performance of the x-ray system being studied. In an embodiment, the MTF is calculated by subtracting from the high average peak the low average peak, then dividing the difference by the sum of the high average peak and the low average peak: [(high average peak)−(low average peak]/[(high average peak)+(low average peak)].

In an embodiment, the plot 122 represents the MTF (frequency) on the Y-axis plotted against the frequency on the X-axis, taken from display 84 of the user interface shown in FIG. 6. The Y-axis is a scale from 1 to 100, with the 100 value representing the ratio of bright and dark signals from rods (e.g. rods associated with group 38 in FIG. 1) having the greatest thickness, shown as a sine wave (peak 38D in FIG. 6). The other points on plot 122 represent ratios for rods of other thicknesses, i.e., from sine waves 40D-48D in FIG. 6. The X-axis represents the frequency in terms of number of rods per unit width, so as the rods decrease in thickness, the frequency of rods per unit width increases. This plot 122 is used to record, monitor, and compare x-ray backscatter system performance. In an embodiment, the display 14 also shows a reference plot 123 of a reference MTF on the user interface of image 80 of FIG. 6.

The reference plot 123 is kept in storage 76 (FIG. 1) of readings of the system 10 taken as initial readings when the system has been adjusted and when all components are properly calibrated. As shown in FIG. 7, the plot 122 shows that the system 10 has degraded slightly from the reference settings, because the curve shows slightly lower values at each frequency.

As shown in FIG. 8, the process for testing the 10 using the reference standard 12 and user interface 16 is shown at 126. As shown in step 128, an everyday system MTF measurement is taken using the reference standard 12 and user interface 16 (FIG. 1). As indicated in block 130, the initial data is taken, and in step 132 the MTF data is captured from the reference sample, such as the image in FIG. 3. This information is stored in storage 76, as indicated at step 134 as a data file and/or a text file. The MTF software is then loaded, as indicated at step 136 and the raw data tab 120 (FIG. 6) is selected, as indicated at step 138.

As indicated at step 140, an MTF scanned image, such as the image of FIG. 4, is selected, and, as indicated at step 142, the photon counts are stored in a data and/or text file.

As indicated at step 144, a region of interest (ROI) or line segment is selected to yield a line profile 82 of image 80 of FIG. 6. In the next step, indicated at 146, the peaks of the line profile are found by actuating the find peaks button 96 on the display of FIG. 6. In step 148, the ROI is selected and added to the MTF, as indicated at step 150. As indicated in step 152, the MTF is plotted, yielding a plot such as plot 122 of display 14 of FIG. 7.

As indicated at step 154, the reference is loaded and a comparison is made between the reference plot and the plot from the selected scanned image taken at step 140, as indicated at step 156. As indicated at decision diamond 158, if the reference plot and the test plot are different, then, as indicated at step 160 the system parameters are adjusted, or the system evaluated to determine the reason for disparity. In the alternative, as indicated at block 162, if the reference plot and the project plot are the same; that is, they coincide, as shown in FIG. 7, the test is completed.

The disclosed system and method for quantifying x-ray backscatter performance solves the problem of the need for measuring, tracking and comparing the imaging capability of x-ray backscatter systems. The system and method may achieve this by their ability to resolve small features with adequate image contrast. Spatial frequency response of an imaging system is the contrast at a given spatial frequency. Spatial frequency is measured in cycles or line pairs per millimeter (lp/mn), which is analogous to cycles per second (Hz) in audio systems. Lp/mn, or cycles per pixel (c/p) or line widths per picture height (LW/PH) can all be used. High spatial frequencies correspond to fine image detail. The response of imaging system components tends to roll off at high spatial frequencies (i.e., smaller features). High spatial frequencies correspond to fine image detail. The more extended the response, the finer the detail and the sharper the image.

The disclosed system and method utilizes a special reference standard in the form of the reference standard 12, and a modulation transfer function (“MTF”) that enables the performance of x-ray backscatter systems to be quantified, and thus compared against a reference standard. Variations in backscatter x-ray intensity across the image of the standard are quantified and displayed on the user interface 16.

As shown in FIGS. 9A and 9B, the system 10 (see FIG. 1) is utilized with an x-ray tube 200, in place of x-ray tube 22, having an aperture 202 that emits an x-ray field of view 204. The x-ray tube 200 is positioned within an enclosure in the form of an aperture wheel 206 having a generally cylindrical housing 208 that encloses the x-ray tube partially or completely. The aperture wheel 206 has a plurality of spokes 210, 212, 214, 216, 218 projecting radially outward from the housing 208. Each of the spokes 210-218 has a hollow interior passage 220, 222, 224, 226, 228, respectively. The hollow interior passages 220-228 is positioned to be rotated into registration with an opening 230 in the housing 208 that allows a collimated x-ray beam 232 from the x-ray tube 200 to exit the aperture wheel 206.

In an embodiment, each of the spokes 210-218 includes an adjustable aperture 234, 236, 238, 240, 242, respectively, mounted on an end of the spoke and in communication with the respective hollow interiors 220-228. As will be discussed, the adjustable apertures 234-242 is selectively adjusted by an operator, or selectively adjusted automatically, to vary the shape and/or size of the opening, which shapes the collimated x-ray beam 232, as well as vary the flux or intensity of the x-ray beam 232 exiting the spokes 210-218. In an embodiment, only one of the apertures 234-242 are adjustable, and the remaining apertures are fixed; that is, the remaining apertures are fixed and non-adjustable in the size of their respective openings.

The aperture wheel 206 includes a motor 244 that is connected by a drive linkage such as a mechanical linkage 246 to rotate the spokes 210-218 to bring successive spokes into registry with the opening 230 to perform a raster scan of an object 247 (FIG. 10) to be inspected.

As shown in FIG. 10, in another embodiment, the x-ray tube 200 is located and wholly or partially enclosed within an enclosure in the form of an enclosure 248 having adjustable apertures 234, 236, 238, 240, and 242. The adjustable apertures 234-242 are mounted directly on the housing 250, rather than on the ends of spokes 210-218 as shown in FIGS. 9A and 9B. The enclosure 248 includes openings 252, 254, 256, 258, 260 that are in registration with the adjustable apertures 234-242 mounted on the housing to allow a collimated x-ray beam 232 to exit the enclosure 248 from the interior 262 of the enclosure 248. In the embodiments of FIGS. 9A, 9B and 10, the x-ray beam 232 is collimated and result from the x-ray field of view 204 emanating from the x-ray tube 200. The enclosure 248 takes the form of a wheel similar in design and operation to the housing 208 of FIGS. 9A and 9B.

The housing 250 is driven by a motor 244 through a mechanical linkage 246 to rotate to bring the adjustable apertures 234-242 successively into alignment with the x-ray field of view 204, so that collimated x-ray beams 232 successively emanate from the adjustable apertures 234-242. The motor 244 is actuated to rotate the enclosure 248 to perform raster scans 263 on the object 247. In embodiments, the object 247 is a body panel or a structural member of a vehicle 249, such as an aircraft, a spacecraft, a land vehicle, or a marine vehicle. In specific embodiments the object 247 is a section of skin of a fuselage or of a wing of a vehicle 249 such as an aircraft, and includes rivets. The x-ray backscatter from the raster scans 263 is measured by x-ray backscatter detectors 18, 20 (see FIG. 1) associated with the x-ray tube 200 and stored in storage 76 and/or displayed on a user interface 16 of display 14.

As shown in FIG. 11, an exemplary adjustable aperture 264 includes two flat, plate-shaped shutters 266, 268 that are positioned by a motor 270, which in embodiments is a small electric motor or a nanomotor. The motor 270 is connected to drive, or include, a threaded shaft 272 that threads into a boss 274 that is connected to the shutter 268. The motor 270 is mounted on a fixed support, such as fixed shaft 276, that is held in place by a boss 278 attached to the fixed shutter 266, or in an embodiment, to adjacent fixed structure such as the enclosure 248 (FIG. 10), or a spoke 210-218 (FIG. 9A). The two shutters 266, 268 slide against an aperture wall segment 280 of the end of, for example, the spoke 210-218 of the enclosure or aperture wheel 206 of FIGS. 9A and 9B, or is a portion of the enclosure 248 of FIG. 10. Alternatively, the adjustable aperture is mounted on an end of x-ray tube 22 (FIG. 1).

In one particular embodiment, each of the shutters 266, 268 is in the shape of a parallelogram. The shutters 266, 268 are positioned in partially overlapping relation to each other, having straight edges or sides 282, 284 that are oriented at an angle of approximately 60° relative to each other, and at an angle of approximately 60° to the adjacent straight side 286 of the aperture wall segment 280 of the housing 208 of the aperture wheel 206 or the enclosure 248. As a result of this orientation, the opening 288 formed by the sides 282, 284, 286 maintains the same shape, which in an embodiment is an equilateral triangle, even as the shutter 268 is moved to the left or right as indicated by the arrow A in FIG. 11 relative to shutter 266 and to the aperture wall segment 280.

Movement of the shutter 268 to the left in FIG. 11 in the direction of arrow A by the motor 270 causes the opening 288 to decrease in size, but maintains its shape of an equilateral triangle. Conversely, movement of the shutter 268 to the right in the direction of arrow A by motor 270 causes the opening 288 to increase in size (i.e. cross-sectional area) while maintaining its shape, which in this embodiment is an equilateral triangle shape. Movement of the shutter 268 relative to fixed shutter 266 and aperture wall segment 280 to vary the size of the opening 288 in this fashion results in increasing or decreasing the flux or intensity of the x-ray beam 232 emanating from the housing 208 or enclosure 248.

Actuation of the motor 270 to vary the size of the opening 288 is effected by a computer control 289, which in embodiments also actuates the motor 244 to cause the x-ray tube 200 and aperture wheel 206 or enclosure 248 to perform raster scans on the object 247. In embodiments, the computer control 289 actuates the motor 270 on the fly; that is, during a raster scans 263 (FIG. 10), to vary the x-ray flux in the x-ray beam 232 directed at the object 247 being inspected. In other embodiments, the x-ray tube 200 and aperture wheel 206 or enclosure 248 is actuated by motor 244 to complete raster scans 263 of an area on the object 247 at one aperture size, for example an opening 288 having an average width of 2 mm.

The computer control 289 then actuates the motor 270 to adjust the size of the opening 288 of the adjustable aperture 264 to a smaller area, for example an opening having an average width of 0.5 mm. The motor 244 then actuates the aperture wheel 206 or enclosure 248 to perform a second, subsequent raster scan 263 of that same area of the object 247 using more scan lines than the previous scan and at a lower x-ray flux or intensity, which may provide a higher resolution, higher contrast scan to be detected by x-ray backscatter detectors 18, 20 (FIG. 1). In an embodiment, the computer control 289 actuates the motor 270 to decrease the average width of the opening 288 further, for example to 0.25 mm, and the computer control 289 actuate the motor 244 to cause the x-ray tube 200 and aperture wheel 206 or enclosure 248 to perform a third, subsequent raster scan 263 of that same area of the object 247 using more scan lines than the previous scan and at still lower an x-ray flux or intensity, which may provide an even higher resolution scan than the two previous scans.

This procedure is desirable in a process wherein the first raster scan 263 of the object 247 finds an anomaly on or in the object, which is found by the x-ray backscatter detectors 18, 20 receiving the x-ray backscatter, and shown on the user interface 16 of the display 14. Then, the x-ray tube 200 is directed by motor 244 and configured by computer control 289 to perform subsequent raster scans 263 of the anomaly on the object 247 at increasingly higher resolutions (i.e., using openings 288 made successively smaller by motor 270). The successive raster scans 263 are performed using a single adjustable aperture, such as the adjustable aperture 264 shown in FIG. 10, which is adjusted during or after each raster scan 263. Alternatively, the successive raster scans 263 are performed using two or more multiple adjustable apertures 234-242 mounted on, for example, the spokes 210-218 of aperture wheel 206, or enclosure 248, wherein the adjustable apertures are adjusted on the fly to openings 288 of successively smaller areas.

As shown in FIG. 12, an adjustable aperture 290 is configured to have two flat, plate-shaped moving shutters 292, 294, each having a projecting boss 296, 298 that is engaged with a respective threaded shaft 300, 302 operably connected to a motor 304, which may be a nanomotor. In embodiments, the motor 304 is fixed to the spoke 210-218, or to the enclosure 248, or the shafts 302 are reverse threaded and/or counter-rotated by the motor 304. The motor 304 is actuated by an operator to selectively move the shutters 292, 294 in the direction of arrow B to increase or decrease the size of the opening 306.

In an embodiment, shutter 294 includes an edge 308 that is positioned at a 60° angle to the edge 310 of an adjacent portion of the wall 313 of the end of a spoke 210-242 (FIG. 9A), for example, and shutter 292 includes an edge 308 that is oriented at a 60° angle to the edge 310, and with respect to edge 312. Thus, edges 308, 310, 312 meet to form an equilateral triangle-shaped opening 306. Shutters 292, 294 overlap so that actuation of the shafts 300, 302 by the motor 304 causes the shutters to variably overlap each other as they move toward or away from each other in the direction of arrow B, either toward the center of the adjustable aperture 290, or away from the center of the adjustable aperture. An advantage of the adjustable aperture 290 of FIG. 12 over the opening 288 of FIG. 11 is that the opening 306 remains centered relative to the apertures 252-260 of the enclosure 248 of FIG. 10, or with the hollow interiors 220-228 of the spokes 210-218 of the housing 208 of FIGS. 9A and 9B.

In a specific application of the adjustable aperture 264, as shown in FIGS. 13A and 13B, the adjustable aperture 264 is mounted on an end 314 of a spoke 316, which may be one or more of the spokes 210-218 of FIGS. 9A and 9B. The end cap 318 is attached to the spoke 316 by a suitable adhesive, pressing it into the spoke, or by mechanical fasteners such that it is removable from the spoke. The spoke 316 includes an end cap 318 having an opening 320 in registration with the opening 288 formed by the adjustable aperture 264. The end cap 318 is shaped to cover and enclose the motor 270 and shafts 272, 276, and partially cover and enclose the shutters 266, 268. The opening 320 is in registration with the opening 288 so that the collimated x-ray beam 232 exits the spoke 316. The opening 320 is shaped to equal or exceed the largest size of the opening 306 of the adjustable aperture 264.

As shown in FIGS. 14A and 14B, a spoke 316 includes an end cap 319 shaped to cover and enclose the adjustable aperture 290. The end cap 319 is attached to the spoke 316 by a suitable adhesive, pressing it into the spoke, or by mechanical fasteners such that it is removable from the spoke. The end cap 319 includes an opening 321 shaped and positioned to be in registration with the opening 306 formed by the adjustable aperture 290. The end cap 319 partially covers not only the overlapping shutters 292, 294 but completely covers the motor 304 and shafts 300, 302. In contrast to the end cap 318 of FIGS. 13A and 13B, where the opening 320 is offset from the center of the end cap to accommodate the shifting position of the opening 288 as the shutter 268 is moved in the direction of arrow A for adjustment of size, the opening 321 is positioned substantially midway in the center of the end cap 319 because the opening 306 remains centrally located as the result of symmetrical movement of the overlapping shutters 292, 294 of the adjustable aperture 290.

As shown in FIGS. 15A, 15B, 15C, and 15D, in another embodiment, the adjustable aperture includes a pair of substantially flat, plate-shaped fixed shutters 322, 324 that are pressed into the open end 332 of a spoke 334, such as a square spoke. The shutters 322, 324 are shaped and positioned on the open end 332 to form a gap or adjustable aperture 326 between them that are filled partially with a shim 328, such as a flat shim, which also are pressed into the open end 332. The shim 328 has a slit 330 formed therethrough. In an embodiment, the shutters 322, 324 and shim 328 are pressed into the open end 332 of a spoke 334 of, for example, an aperture wheel 206 (see FIG. 9A). In embodiments, the slit 330 are oblong and generally rectangular, as shown in FIG. 15A. In other embodiments, the slit are shorter, such as slit 336 of shim 338 in FIG. 15B, or almost square and relatively small, such as slit 340 of shim 342 in FIG. 15C. In other embodiments, the shutters 322, 324, and shims 328, 338, 342, 346 are attached to the open end 332 by a suitable adhesive, by a mechanical connection, such as by screws, or by welding or brazing.

In yet another embodiment, the orientation of the slit 330, 336, 340 are vertical or substantially vertical, as shown in FIGS. 15A, 15B, and 15C, or horizontal, as shown for slit 344 of shim 346 in FIG. 15D, relative to the direction of raster scan 263. For inspections such as stress corrosion crack detection, there is a need for improved resolution along the linear scan direction. Smaller circular apertures may help, but as the aperture becomes very small, the whole is difficult and expensive to machine accurately, and also may limit the x-ray flux and therefore slow down a backscatter x-ray scan to a point where it is impracticable to perform.

By providing the shims 328, 338, 342, and 346 with slits 330, 336, 340, and 344, respectively, press fit into, or otherwise attached to, the open ends 332 of spokes 334, the cost of manufacture is reduced and the design is simplified of the aperture wheel 206 or enclosure 248. In addition, the slits 330, 336, 340, 344 allow for increased resolution, due to the narrow slit, but an increased flux due to the relatively wider opening, so scan speeds remain high enough to be practicable. While vertical resolution is reduced slightly, horizontal resolution, which is important for crack detection when their orientation is generally predictable, may be improved over prior art apertures having a uniform cross-section.

Similarly, as shown in FIGS. 16A, 16B, 16C, and 16D, the shutters 348, 350 are substantially flat and plate-shaped, having a generally semi-circular shape. The shutters 348, 350 are press-fitted into an open end 352 of a spoke 354 of an aperture wheel, such as a circular spoke of the aperture wheel 206 shown in FIG. 9A, or enclosure 248 of FIG. 10. The shutters 348, 350 are shaped to form a gap 356 when pressed into or otherwise attached to the ends 352 of the spokes 354, and a shim 358 are fitted between the shutters 348, 350 within the gap. The shim 358 has the slit 360 therethrough of a predetermined width and length, preferably having a length that is a multiple of the dimension of the width to allow increased resolution, similar to slit 330 in FIG. 15A. Similarly, in FIG. 16B, shim 362 has a slit 364, and in FIG. 16C, shim 366 has a slit 368, and in FIG. 16D, shim 370 has a slit 372. The orientation of the slit 372 in FIG. 16D is at an angle to facilitate detection of a stress corrosion crack of a known or expected orientation.

As shown in FIG. 17, an adjustable aperture 374 includes an outer pair of substantially flat, plate-shaped shutters 376, 378 having bosses 380, 382, respectively, threaded into a shaft 384 that is rotated by a motor 386, which in embodiments is a nanomotor. The adjustable aperture 374 also includes an inner pair of shutters 388, 390 that are substantially flat and plate-shaped, and include bosses 392, 394, respectively, threaded into a shaft 396 that is rotated by a motor 398, which in embodiments also is a nanomotor. The outer pair of shutters 376, 378 and the inner pair of shutters 388, 390 is mounted on an end 399 of a spoke 316 (FIG. 14B) of an aperture wheel 206 (FIG. 9A), or mounted on the enclosure 248. The shafts 384, 396 are threaded such that, when rotated by the motors 386, 398, respectively, the outer shutters 376, 378 and inner pair of shutters 388, 390 each move toward each other or away from each other. The motors 386, 398 likewise are attached to the end 399 of the spoke 316.

The inner pair of shutters 388, 390 are oriented at an angle relative to the outer pair of shutters 376, 378, and in a particular embodiment are oriented at a 90° angle to the shutters 376, 378. The outer pair of shutters 376, 378 are positioned adjacent the inner pair of shutters 388, 390, and in an embodiment immediately behind them, to form with them a slit 400 that are defined by an edge 402 of shutter 388, an edge 404 of shutter 390, an edge 406 of shutter 376, and an edge 408 of shutter 378. The motors 386, 398 are actuated by a control 410, which also actuates motor 244 (FIGS. 9A and 10) so that the motors 386, 398 displace the shutters 376, 378 relative to each other, and the inner pair of shutters 388, 390 relative to each other, so that the shape and open area of the slit 400 are varied as desired. Further, this adjustability of the aperture opening are effected during a scan, if desired. The adjustable aperture 374 is mounted on an end of a spoke, such as one or more of the spokes 210-218 in FIG. 9A, or mounted on the enclosure 248 of FIG. 10. In another embodiment, a single adjustable aperture 374 is mounted on an end of x-ray tube 22 (FIG. 1).

As shown in FIGS. 18A and 18B, in another embodiment, adjustable aperture 412 includes an outer pair of adjustable shutters 414, 416, and an inner pair of adjustable shutters 418, 420, which are mounted on an end 399 of a spoke 316 of an aperture wheel 206 or the x-ray tube 22, or mounted on enclosure 248. Shutters 414, 416 are attached to piezoelectric elements 422, 424 that are connected to voltage sources 426, 428. The voltage sources 426, 428 are actuated by a control 430 so that the spacing between the shutters 414, 416 are varied by actuating the piezoelectric elements 422, 424. In an embodiment, a second pair of piezoelectric elements 432, 434 are attached to and positioned between adjustable shutters 418, 420. Piezoelectric elements 432, 434 are actuated by voltage sources 436, 438 controlled by control 430. Thus, piezoelectric elements 422, 424 control the shape, size, and aspect ratio of the slit aperture 440, which are defined by the edge 442 of one of the adjustable shutters 414, edge 444 of one of the adjustable shutters 416, edge 446 of one of the adjustable shutters 418, and edge 448 of one or the adjustable shutters 420.

As shown in FIG. 18B, actuation of piezoelectric elements 422, 424 by control 430 cause the shutters 414, 416 to move in the direction of arrows C, thus increasing the width of the slit aperture 440 of the adjustable aperture 412. Similarly, control 430 actuate voltage sources 436, 438 (shown in FIG. 18A) to activate piezoelectric elements 432, 434 to move adjustable shutters 418, 420 toward or away from each other to vary the shape of the slit aperture 440. This actuation of piezoelectric elements 422, 424, 432, 434 are done selectively and without removing the adjustable aperture 412 from the x-ray tube 22 or enclosure 248 on which it is mounted. Further, the control 430 are programmed to vary the shape (including the aspect ratio) of the slit aperture 440 in a pre-programmed fashion.

Thus, with the embodiment of FIGS. 18A and 18B, a slit type adjustable aperture 412 is adjustable on the fly using piezoelectric materials. The piezoelectric materials expand or contract based upon an applied voltage across selected crystal orientations. This structure and associated method is used for the adjustable apertures shown in FIGS. 9A-17 as well.

The adjustable apertures 234-242, 264, 290, 326, 338, 342, 346, 358, 362, 366, 370, 374, and 412 shown in FIGS. 9A-18B preferably are made of tungsten. These adjustable apertures and associated method provide an improved set of adjustable apertures for an x-ray backscatter system that controls the size and shape of the x-ray beam that is rastered across the target structure (i.e., the object 247) during a raster scan 263. These adjustable apertures improve system performance by providing options beyond the standard circular opening of selected size machined into tungsten. These apertures are adjustable to reduce the need for multiple sets of apertures, each of a different, fixed size, and can provide improved resolution of defects in selected directions without sacrificing flux or scan speed.

These disclosed adjustable apertures are adjusted, either using the motors 270, 304, 386, 398, or by replacing the shims 328, 338, 342, 346, 358, 362, 366, and 370 to provide an opening that is very small, on the order of 0.25 mm or 0.5 mm in width in order to detect small cracks such as stress corrosion cracks. At the same time, and if desired, on the fly, the adjustable apertures are adjusted to have a larger aperture that allows for general scanning at higher flux levels, which enables higher scan speeds. This can be done without replacing the entire aperture to achieve the desired size. The adjustable apertures eliminate the need for software normalization to achieve consistent x-ray backscatter flux intensity. Further, the orientation of the apertures is selected such as the slits 344, 372 in FIGS. 15B and 16B, or adjusted, such as the adjustable aperture 374 of FIG. 17 and the adjustable aperture 412 of FIGS. 18A and 18B to provide improved crack detection that results from orienting the wide dimension of the aperture perpendicular aperture. Other orientations of the aperture relative to the predicted crack orientation are selected to improve the image resolution and defect detection.

While the methods and forms of apparatus herein described constitute preferred embodiments of the disclosed system and method for quantifying x-ray backscatter system performance, it is to be understood that variations may be made therein without departing from the scope of the disclosure. 

What is claimed is:
 1. A system for quantifying x-ray backscatter system performance, the system including one or more x-ray backscatter detectors and an x-ray tube, the system comprising: a support; a plurality of rods mounted on the support and arranged in groups, wherein each group of rods include at least two rods having the same width; and a user interface configured to connect to the one or more x-ray backscatter detectors to receive a backscatter signal from the one or more x-ray backscatter detectors associated with the x-ray tube, wherein the user interface plots a modulation transfer function representing x-ray backscatter for each rod of the plurality of rods from x-rays transmitted by the x-ray tube.
 2. The system of claim 1, further comprising a storage in communication with the user interface, wherein a reference modulation transfer function plot representing initial readings representing reference settings of the system are kept in the storage.
 3. The system of claim 2, wherein the user interface includes a display, and wherein a plot representing the modulation transfer function and the reference modulation transfer function plot are both shown upon the display.
 4. The system of claim 1, wherein the user interface is configured to display a backscatter image based on the backscatter signal from the one or more x-ray backscatter detectors.
 5. The system of claim 4, wherein the each group of rods includes varying widths, and wherein the plurality of rods are arranged to become progressively become thinner in width.
 6. The system of claim 5, wherein a portion of the backscatter image corresponding to a group of thicker rods is more sharply defined when compared to another portion of the backscatter image corresponding to a group of thinner rods.
 7. The system of claim 4, wherein the user interface is configured to display an image of a line profile representing photon counts of the backscatter image.
 8. The system of claim 1, wherein the modulation transfer function includes a ratio of a high average peak of photon counts minus a low average peak of photon counts from the x-ray backscatter, divided by a sum of the high average peak of photon counts plus the low average peak of photon counts.
 9. The system of claim 1, wherein the plurality of rods are arranged parallel to each other.
 10. The system of claim 1, wherein the plurality of rods each have curved outer surfaces.
 11. The system of claim 1, wherein the plurality of rods are made of a polymer.
 12. The system of claim 1, wherein the user interface represents a modulation transfer function frequency on a y-axis, and wherein a frequency in terms of rods per unit length is represented by an x-axis.
 13. A method for quantifying x-ray backscatter system performance, the method comprising: receiving, by a user interface, a backscatter signal from one or more x-ray backscatter detectors, wherein the backscatter signal indicates x-ray backscatter for a plurality of rods mounted on a support and arranged in groups, wherein each group of rods include at least two rods having the same width; and plotting, by the user interface, a modulation transfer function representing the x-ray backscatter for each rod of the plurality of rods from x-rays transmitted by an x-ray tube.
 14. The method of claim 13, further comprising: showing a reference modulation transfer function plot on a display, wherein the reference modulation transfer function plot is part of the user interface representing initial readings representing reference settings.
 15. The method of claim 14, further comprising: plotting the modulation transfer function on the display; and comparing the reference modulation transfer function plot with the modulation transfer function to determine the x-ray backscatter system performance.
 16. The method of claim 13, further comprising: displaying, by the user interface, a backscatter image based on the backscatter signal from the one or more x-ray backscatter detectors.
 17. The method of claim 16, further comprising: displaying a portion of the backscatter image corresponding to a group of thicker rods more sharply when compared to another portion of the backscatter image corresponding to a group of thinner rods.
 18. The method of claim 13, further comprising: displaying, by the user interface, an image of a line profile representing photon counts of the backscatter image.
 19. The method of claim 13, further comprising calculating the modulation transfer function by: subtracting, from a ratio of a high average peak of photon counts, a low average peak of photon counts from the x-ray backscatter to determine a difference; and dividing the difference by a sum of the high average peak of photon counts plus the low average peak of photon counts.
 20. The method of claim 13, wherein the user interface includes an x-axis and a y-axis, and wherein the y-axis represents a modulation transfer function frequency and the x-axis represents a frequency in terms of rods per unit length. 